A typical ink jet printer, plotter, or other printing system has a pen that reciprocates over a printable surface such as a sheet of paper. The pen includes a print head having an array of numerous orifices or nozzles through which droplets of ink may be expelled into the surface to generate a desired pattern. Color ink jet printers typically employ four print heads, each connected to an ink supply containing a different color of ink (e.g. black, cyan, yellow, and magenta.) The different print heads may be included on separate, replaceable ink pens. A full color image may be printed by sequentially or simultaneously printing overlapping patterns with each of the different color inks. For good printed output, the patterns of different printed color images must be in precise registration. Registration errors occur because the print heads may differ in dimension by slight tolerances, and because removal and installation may prevent print head positioning from being precisely repeatable. At a typical 600 dots per inch (dpi) printer resolution, errors by more than one dot pitch are considered unacceptable.
In existing printers, registration of the different colors may be achieved without user involvement by printing an alignment pattern with each color, then visually or optically sensing the positions of the printed patterns and determining the amounts of any deviations from nominal aligned positions. The printer may then electronically adjust the firing position or timing for each color so that the resulting output is aligned. This is particularly critical for plotters printing on large media sheets, in which small errors may accumulate to provide unacceptable output.
For lower cost printers desired by many users, a vernier alignment pattern is printed, the user visually identifies which of several different black and color patterns is best aligned, and then enters the information into his computer or printer. A vernier alignment pattern is printed with a sequence of thin, equally spaced black lines serving as reference rulings. Adjacent to this pattern, a sequence of similar color lines is printed, except with a slightly wider or narrower spacing. A central one of the color lines is printed at a position nominally aligned with the corresponding black line, while each of the adjacent color lines is shifted from the nominal by a single dot pitch in opposite directions, and each further removed color line is shifted by an increased integral multiple of the dot pitch. While the black lines are spaced apart by n pixels, the color lines may be spaced apart by n+1 (or n-1) pixels.
If the pens are in proper alignment, the central color line will appear best aligned. If the pens are misaligned by "n" dot pitches in a given direction, the line pair located n units away from the central pair will appear best aligned. By identifying this pair, the user can instruct the printer to shift the print data in time, or to shift which nozzles to which the print data will map, to correct the error. This proceeds with each color, using black as the reference in each case, and includes rows of lines oriented in each orthogonal axis, to detect and correct misalignments in the scan axis and the feed axis of the printer. Scan axis errors are corrected by shifting the timing of printing the color droplets; feed axis errors are corrected by shifting the nozzles to which the print data corresponds.
While reasonably effective, the vernier alignment system requires critical visual acuity and skill that may be inadequate in some users. Also, chromatic aberrations caused by corrective eyewear and an imperfect viewing axis may cause an illusory shifting of different colors relative to the black reference lines. Most difficult is the alignment of the yellow ink used in typical four-color printers. A fine yellow line is difficult to discern on normal white printer paper, as there is insufficient contrast between the bright yellow figure and the bright white background. In addition, for vertical alignment (using horizontal lines parallel to the scan axis) only a single nozzle is used to print each color. Thus, an error on one nozzle could lead to a misalignment of the rest of the nozzles of that color.
The present invention addresses the disadvantages of the prior art by providing an apparatus and method of aligning different color print heads of an ink jet printer by printing a sequence of extended-area first alignment elements in a first color, then printing an overlaying sequence of extended-area second alignment elements in a second color. Each of the alignment elements includes an array of spaced apart printed lines. The first alignment elements are printed to nominal reference positions, and the second alignment elements are offset from the reference positions by differing amounts. The degree of overlap of the first and second color lines is readily visible, and the most thoroughly overlapped alignment element may be readily identified. The identity of this element may then be fed back to the printer to respond with electronic alignment measures.